Angiotensin converting enzyme, an important component of the renin-angiotensin system, will be isolated from rabbit lung and other tissues by conventional procedures and by affinity chromatographic methods to be developed. The enzyme will be characterized in terms of its metal content and functional amino acid residues at its active center. A new assay procedure will be developed and employed to determine the kinetic properties of converting enzyme and their relationship to those of other metallopeptidases such as carboxypeptidase A and thermolysin. The catalytic mechanism will be investigated by isotopic labeling experiments using mass spectrometry, by selective inhibitors using stopped-flow and steady-state kinetics, by substituting a series of different metals for the intrinsic metal atom, and by determining the physicochemical basis for chloride ion activation. The results should provide new information on the mode of action of converting enzyme and greater understanding of its role in blood pressure regulation.